The present invention relates generally to a method for reducing the content of certain materials in molten steel, by the employment of "vacuum degassing," and more particularly to a method in which oxygen blowing is employed in conjunction with vacuum degassing. Examples of such methods are the Ruhrstahl-Heraeus (RH) process or the "RH-OB" process wherein "OB" means oxygen blowing.
Vacuum degassing is a process which includes exposing molten steel to a low-pressure environment (e.g., a vacuum) in order to remove certain materials from the steel in the form of gases. Examples of gaseous materials which can be removed from the molten steel include hydrogen, oxygen, carbon monoxide, nitrogen, and sulfur. The material to be removed from the molten steel may also be a derivative of, or precursor to (e.g., an elemental component of), these gases; for example, carbon may be removed from steel in the form of carbon monoxide and/or carbon dioxide. Materials such as those listed above (or their derivatives or precursors), unless removed, can cause porosity, flaking, embrittlement, voids, inclusions, and other undesirable conditions in the steel after it is solidified.
A vacuum degassing process operates on the principle that reducing the pressure of (e.g., providing a vacuum treatment to) a closed environment surrounding molten steel will reduce the partial pressure of a gas derived from (e.g., produced by a chemical reaction in) that molten steel. Reducing the partial pressure of the gas will improve the thermodynamics of a reaction which releases the gas from the molten steel. The thermodynamics improve because a natural equilibrium state will attempt to reestablish itself between (a) the gas in the environment above the molten steel and (b) the gas present (e.g., in a dissolved state or in the form of the elements which react to form the gas) in the molten steel. There are three basic types of vacuum degassing: spray degassing, enclosed ladle degassing, and recirculation degassing.
As stated above, it may be desirable to remove hydrogen, for example, from molten steel by vacuum degassing. In such a process, the pressure in the environment above the molten steel is lowered by exhausting therefrom hydrogen and any other gas present in the environment above the molten steel. Removal of hydrogen gas reduces the partial pressure of hydrogen in the environment above the molten steel, thereby shifting the equilibrium between (a) the hydrogen in the molten steel and (b) the hydrogen in the environment above the molten steel. As the process continues, the molten steel attempts to regain equilibrium by evolving hydrogen into the environment above, from which one may continue to exhaust hydrogen. This same principle may be utilized in order to remove other materials, such as oxygen and carbon, from the molten steel (e.g., as carbon monoxide gas), as further described below.
Stirring the molten steel bath facilitates the above-described mechanism because stirring causes the materials which are to be removed to rise from lower locations of the molten steel bath nearer to the surface of the molten steel, where these materials can be more readily removed from the molten steel into the atmosphere. For example, in the process known as ladle degassing, a heat of molten steel is placed into a ladle which is in turn placed inside a vacuum chamber, wherein the metal is exposed to low pressure and stirred either by a gas or by electrical induction.
In another type of vacuum degassing process, known as recirculation degassing (also referred to as circulation degassing), a stream from the main bath of molten steel is initially forced by atmospheric pressure into an evacuated degassing or treatment chamber wherein the steel there is exposed to low pressure. After exposure to low pressure, the molten steel flows out of the degassing chamber, and back to the main bath.
A Ruhrstahl-Heraeus or "RH" process (a type of recirculation degassing) typically employs a vertically disposed treatment vessel, downwardly from the bottom of which extend a pair of tubular members or conduits having open lower ends and which function as siphon tubes or "snorkel" tubes. This treatment vessel is preferably disposed directly above a ladle which contains a bath of molten steel covered with slag and including dissolved carbon and oxygen. This bath of molten steel is supplied from a primary steelmaking stage, such as from a basic oxygen furnace, electric arc furnace, or open hearth furnace. The ladle is raised until the lower open ends of the two snorkels extend below the slag layer which rests on top of the molten steel. The interior of the treatment vessel is evacuated through an exhaust outlet located near the top of the vessel, creating a sub-atmospheric low pressure environment in the vessel. The atmospheric pressure on the molten steel in the ladle causes the molten steel and slag cover in the snorkels to rise upwardly through the two snorkels into the interior of the treatment vessel.
A Dortmund-Horder-Huttenunion or "DH" system is similar to an RH system, but differs in that the apparatus has only one snorkel, through which the molten steel both enters and exits the treatment vessel.
Generally in an RH system, stirring gas, such as argon or nitrogen, is introduced into a first snorkel to reduce the density of the molten steel therein. (Gas stirring can also be used in other degassing systems in which stirring is beneficial, such as DH systems and ladle degassing.) The effect of the differential in densities is that the relatively less dense molten steel enters the treatment vessel through the first snorkel (known as the "up leg" or "inlet snorkel") and the relatively denser molten steel exits the treatment vessel through the other snorkel (known as the "down leg" or "exit snorkel").
In the manner described above, the molten steel may be circulated from the ladle upwardly through the inlet snorkel into the interior of the treatment vessel. Once the molten steel has been treated in the treatment vessel, the molten steel flows downwardly through the exit snorkel back into the ladle (below the treatment vessel). The molten steel will continuously circulate into, and out of, the treatment vessel to allow for maximum exposure of the steel to the low pressure environment. When the molten steel is in the treatment vessel, it undergoes a process which facilitates the release of various gases from the molten steel (e.g., carbon monoxide and/or hydrogen), thereby reducing the content of such materials in the molten steel.
The introduction of the stirring gas, such as argon or nitrogen, into the molten steel at the inlet snorkel causes circulation of the molten steel, which increases the surface area of molten steel exposed to the reduced pressure within the treatment vessel. This procedure facilitates the treatment for the reasons described above. Such stirring gases are referred to as "lift gases" because these gases circulate the molten steel by causing the steel to rise into the treatment vessel. Use of a lift gas may, in some cases, provide an additional advantage in that the lift gas forms bubbles which can function as a carrier for the "impurity" gas (e.g., hydrogen) which is coming out of solution in the molten steel.
The use of argon as a stirring or lift gas has the disadvantage of being relatively expensive. Nitrogen, although less expensive than argon, is disadvantageous in that some of the nitrogen can dissolve in the molten steel, and for many types of steel it is desirable to limit the nitrogen content. Therefore, nitrogen generally cannot be used as a stirring or lift gas in the manufacture of ultra-low nitrogen grades of steel. Further, nitrogen will enter into the interstices of the lattice structure of solidified steel, which is undesirable because when elements such as nitrogen enter those interstices, the drawing (cold deforming) capabilities of the steel are reduced. Therefore, even for non ultra-low nitrogen grades of steel, the nitrogen content preferably should be below 100 parts per million (ppm). In many ultra-low carbon ("ULC") steels, titanium is added to stabilize the microstructure of the hardened steel. These titanium-stabilized steels, in particular, must generally have a low nitrogen content.
Continuing the above-described circulation procedure for a period of time results in the entire volume of the molten steel in the ladle being subjected to the degassing treatment. The total contents of the ladle, usually an entire heat of steel from a basic oxygen furnace (typically about 250 short tons (227 Mg) of molten steel), is typically circulated through the treatment vessel in about 1.5 minutes. The bath of molten steel in the ladle is typically recirculated through the treatment vessel for as long as it takes to reduce the content of a particular impurity material to a desired level. By way of example only, the total treatment time can be in the range of about 10 to about 50 minutes, depending upon the desired product and the process conditions that are present or employed.
As stated above, it may be desirable to remove carbon from molten steel. If the dissolved oxygen content in the molten steel is sufficiently high, this oxygen will react with carbon in the molten steel to produce carbon monoxide gas. The production of carbon monoxide gas will be limited, however, by the thermodynamics of the following reaction: C+O.revreaction.CO. Because it is desirable to have a high oxygen content in order to remove carbon from the molten steel as carbon monoxide gas (i.e., to force the above reaction to the right), the molten steel subjected to the treatment described above will typically be non-deoxidized before the treatment begins. The term non-deoxidized steel refers to molten steel which contains a substantial quantity of dissolved oxygen which has not been removed by reaction with a deoxidizing agent, such as aluminum or silicon.
Because the resulting carbon monoxide gas is insoluble in molten steel, the carbon monoxide gas escapes from the molten steel into the environment thereabove (which, in a vacuum degassing system, is closed to the outer atmosphere). The carbon monoxide is thereafter removed or exhausted from the enclosed environment through various means, for example the imposition and maintenance of a low-pressure in the environment surrounding the molten steel. This process removes both oxygen and carbon from the molten steel as carbon monoxide gas. Use of such a process, known as natural decarburization, allows production of steels with ultra low carbon contents (e.g., 0.002 wt. %).
As stated above, the reaction between carbon and oxygen to form carbon monoxide (CO) is an equilibrium reaction which can move in either direction, and may be shown as: C+O.revreaction.CO. The ability of the reaction to form carbon monoxide is (a) directly related to the dissolved carbon and oxygen content of the molten steel and (b) inversely related to the partial pressure of carbon monoxide in the atmosphere above the molten steel.
Initially, the non-deoxidized molten steel has a high dissolved oxygen content of about 50 to 600 ppm, for example, so that lowering the partial pressure of carbon monoxide, by exhausting carbon monoxide gas from the enclosed atmosphere above the molten steel, drives the reaction to produce carbon monoxide (i.e., to the right in the equation shown in the previous paragraph).
Further, when carbon monoxide is continuously withdrawn from the environment above the molten steel, thereby maintaining there a relatively low partial pressure of carbon monoxide (e.g., 200 torr or less), the reaction is continuously driven to form carbon monoxide. The net result is to reduce substantially the carbon content (and the dissolved oxygen content) of the molten steel. Generally, the lower the partial pressure of carbon monoxide which is maintained, the lower the carbon content which can be achieved in the molten steel.
The process described above is known as an RH process. A refinement of the RH process is known as an RH-OB process. In an RH-OB process, the treatment vessel is equipped with oxygen tuyeres or blowers in the sides of the vessel, generally at the lower part thereof. Oxygen can be blown through these tuyeres into the molten steel in the treatment vessel, and this provides several potential benefits.
More particularly, the oxygen can be utilized to accelerate decarburization, and this is known as forced decarburization. Forced decarburization provides faster processing of the steel in the vacuum degassing vessel than natural decarburization, which is desirable in that it maximizes utilization of downstream casting equipment, such as a continuous caster which can be scheduled to continuously cast the molten steel from the degassing vessel. Another advantage of forced decarburization is that the untreated molten steel can be tapped from the primary steelmaking furnace at a significantly higher carbon level and a significantly lower dissolved oxygen level than when the molten steel from the basic oxygen furnace is to be subjected to a vacuum degassing treatment which does not employ oxygen blowing (i.e., it employs natural decarburization, instead). Oxygen blowing increases the amount of carbon which can be removed from the molten steel by vacuum degassing at a given sub-atmospheric pressure, and is therefore particularly useful in the production of ultra-low carbon steels. Oxygen blowing also reduces the time period required to reduce carbon to the desired level at a given partial pressure of carbon monoxide.
In addition, in a treatment vessel equipped to provide oxygen blowing, the molten steel undergoing treatment can be reheated employing a process called aluminum reheating in which (1) aluminum is added to the molten steel and (2) oxygen is blown through the tuyeres, causing a reaction between the aluminum and the oxygen. Such a reaction is exothermic and produces heat. Aluminum reheating is disadvantageous in some respects because aluminum is relatively expensive and the aluminum oxide formed by the reaction during aluminum reheating must be flushed from the molten steel into the slag cover on the molten steel, and this requires additional recirculation time which in turn prolongs the process.
Carbon dioxide (CO.sub.2) has been blown into molten steel in a basic oxygen furnace, and, during earlier stages of the primary steelmaking process conducted in that furnace, the CO.sub.2 has helped lower the carbon content. However, the thermodynamics which exist during primary steelmaking limit the reduction in carbon content to no lower than about 0.03 wt. %, for example, when employing CO.sub.2 for that purpose.
In some RH-OB processes, concentric tubular tuyeres are employed for forced decarburization. An inner concentric tube or conduit is used to conduct the tuyere gas (e.g. oxygen); an outer concentric tube or conduit is used to conduct a tuyere protection gas. In some operations, carbon dioxide has been used as a tuyere protection gas.